1. Field of the Invention
This invention relates to an improved method and system for examining a microelectronic wafer, and more particularly, to an improved method and system for measuring the protrusion of a microelectronic wafer relative to a carrier ring used to hold the wafer.
2. Description of the Related Art
The following descriptions and examples are given as background only.
In many cases, elevational fluctuations in the surface of a microelectronic topography may be removed by polishing the topography. A typical polishing process may involve placing a microelectronic wafer against a carrier plate, which is surrounded by a carrier ring adapted to prevent movement of the wafer during polishing. The wafer may be pressed face-down toward an underlying polishing pad. During the polishing process, the polishing pad and/or the wafer carrier may be set in motion as the wafer is forced against the pad. In some embodiments, an abrasive, fluid-based chemical suspension, often referred to as a “slurry,” may be deposited onto the surface of the polishing pad. The slurry may fill the space between the polishing pad and the wafer surface such that a chemical in the slurry may react with the surface material being polished. In addition, the movement of the polishing pad and/or wafer relative to each other may cause abrasive particles entrained within the slurry to physically strip the reacted surface material from the wafer. Alternatively, the slurry may be substantially absent of particulate matter and/or chemicals.
In general, it is beneficial for the wafer to extend from the carrier ring by a certain amount such that the carrier ring does not interfere with the polishing process (i.e., such that the carrier ring does not come into contact with the polishing pad during the polishing process). In particular, the protrusion should be large enough to allow a sufficient amount of slurry to be positioned between the wafer and polishing pad. On the other hand, the protrusion should be small enough such that the wafer is securely arranged within the wafer carrier. Since the thicknesses of the carrier plate and carrier ring may vary due to low tolerance fabrication specifications and/or warping during the polishing process, shims are often placed between the carrier ring and carrier plate in order to obtain an optimum wafer protrusion. As a consequence, the protrusion of a wafer extending from a carrier ring generally has to be measured after each wafer carrier assembly to insure that the shims position the wafer within the process specifications of the polishing system.
Unfortunately, some conventional systems used to measure the protrusion of the wafer relative to a carrier ring are not consistently accurate or precise. For example, a system which includes a distance measurement device mounted to a tripod stand may not offer a manner with which to obtain consistent protrusion measurements. In particular, such a system generally necessitates the placement of the tripod stand upon the carrier ring and wafer, causing the wafer and/or carrier ring to compress. Consequently, the resulting measurement may be distorted from the actual protrusion of the wafer relative to the carrier ring. In addition, such a system typically requires operator interface to place the tripod stand upon the carrier ring and wafer, increasing the likelihood of further compression of the carrier ring and/or wafer. Furthermore, the tripod stand may be generally unstable, since it has only three bases of support and it is placed upon the unlevel surface of the wafer and the carrier ring. Such instability of the tripod stand may undesirably cause the distance of the measurement device relative to the wafer and carrier ring to fluctuate, causing excessive variability in the protrusion measurements of the wafer.
In some embodiments, controlling the protrusion of a wafer relative to a carrier ring may further aid in controlling the polish rate of the polishing process. As stated above, the protrusion of a wafer should be large enough to allow a sufficient amount of slurry to be positioned between the wafer and polishing pad. As such, the protrusion of the wafer may be used to control the amount of slurry that is positioned between the wafer and polishing pad, to thereby control the polish rate of the polishing process. In addition, the protrusion of the wafer may be used to control the uniformity of the polishing process across the wafer. Unfortunately, as stated above, conventional methods for measuring protrusions of a wafer relative to a carrier ring may not produce accurate and precise measurements. As a result, a polishing process in which a protrusion was measured by such conventional methods may not form a substantially planar surface across an entire microelectronic topography. In particular, a microelectronic topography polished by a process in which a protrusion was measured by a conventional method may still have substantial elevational disparities. Such elevational disparities may inhibit the formation of functional microelectronic devices on the microelectronic topography. For example, a thick region of the microelectronic topography may form microelectronic devices with dimensions which deviate significantly from design specifications.
Accordingly, it would be advantageous to develop a method and system for measuring the protrusion of a wafer relative to a carrier ring. More broadly, it would be beneficial to develop a system and method for measuring relative distances upon a microelectronic wafer. In addition, it would be advantageous to develop a system which is adapted to receive and hold a microelectronic wafer without incurring additional weight upon the wafer.